Phototransduction in large identifiable Aplysia neurons is being analyzed at three levels: the nature of the photopigment, the coupling between photon absorption and membrane electrophysiological response, and the membrane response itself. The organelles mediating the response have been isolated and analyzed for pigment and for elemental content. A diffusion-based model has been developed to account for the kinetic responses and the non-linear steady state intensity-response curve. Further studies on the light-evoked release of Ca 2 ion and on the unitary membrane events underlying the response, will be done.